1. Field of the Invention
The present invention relates to tuners, and more particularly, to a tuner that prevents a reference frequency signal from being superposed as a disturbance signal on a local oscillating signal.
2. Description of the Related Art
In CATV, programs for several tens of channels can be transmitted by re-transmitting a broadcasting wave, a vacant channel, a middle band, and a super band. In such CATV, since both signal scrambling and transmission of necessary information are performed to select a program and perform accounting, a double-conversion tuner is placed before a receiver to perform receiving.
FIG. 3 is a block diagram of a tuner related to the present invention. A tuner 50 is provided with an input terminal 1, first to fourth band-pass filters 2a to 2d, an automatic gain controller 3, first to third amplifiers 4a to 4c, first and second mixers 5a and 5b, first and second voltage controlled oscillators 6a and 6b, first and second PLL ICs 7a and 7b, a reference oscillator circuit 8, and an output terminal 9.
The tuner 50 is a double-conversion type, in which the frequency of an input signal is increased by an upconverter and then, decreased by a downconverter to remove disturbance.
The operation of the tuner 50 will be described next. An input signal input to the input terminal 1 is attenuated at frequencies other than those in the frequency band of a desired receiving signal by a first bandpass filter 2a, limited to a predetermined level range by an automatic gain controller 3 according to an automatic-gain-control voltage based on the receiving-signal level, amplified by a first amplifier 4a, and sent to a first mixer 5a. The first mixer 5a converts the input signal to a first intermediate-frequency signal according to a first local oscillating signal output from the first voltage controlled oscillator 6a. In this case, the oscillating frequency of the first voltage controlled oscillator 6a is controlled and stabilized by a first PLL circuit included in the first PLL IC 7a. 
The first intermediate-frequency signal output from the first mixer 5a is attenuated at frequencies other than those in a first intermediate-frequency band by the second bandpass filter 2b, amplified by the second amplifier 4b, attenuated again at frequencies other than those in the first intermediate-frequency band, and sent to the second mixer 5b. The second mixer 5b converts the input first intermediate-frequency signal to a second intermediate-frequency signal according to a second local oscillating signal output from the second voltage controlled oscillator 6b. In this case, the oscillating frequency of the second voltage controlled oscillator 6b is controlled and stabilized by a second PLL circuit included in the second PLL IC 7b. The second intermediate-frequency signal output from the second mixer 5b is attenuated at frequencies other than those in a second intermediate-frequency band by the fourth bandpass filter 2d, amplified by the third amplifier 4c, and output from the output terminal 9.
The first PLL IC 7a and the reference oscillator circuit 8 are directly connected, and the second PLL IC 7b and the reference oscillator circuit 8 are connected through a capacitor C2 and an amplifier AMP. The reference oscillator circuit 8 includes a crystal oscillator X and a capacitor C1, and sends a reference-frequency signal oscillated by a reference oscillating circuit including the reference oscillator circuit 8 and an amplifier included in the first PLL IC 7a, to the first voltage controlled oscillator 6a. The reference-frequency signal is also sent to the second PLL IC 7b through the capacitor C2 and the amplifier AMP.
In the tuner described above, the first voltage controlled oscillator, the reference oscillator circuit, and the first PLL IC, which includes the amplifier for reference oscillation, are located close to each other in a cabinet due to recent demands for compactness. In such a case, the first local oscillating signal flows undesirably into the amplifier from a connection terminal between the reference oscillator circuit and the first PLL IC. The reference-frequency signal is mixed with the first local oscillating signal or the first local oscillating signal is superposed on the reference-frequency signal. The mixed or superposed signal is added to the first voltage controlled oscillator through the first PLL IC. Since the oscillating frequency of a crystal oscillator in the reference oscillator circuit usually falls in the 4-MHz band, the mixed or superposed signal has two frequencies that are equal to that of the first local oscillating signal frequency ±4 MHz. Because the frequencies of the signal fall in an analog-video-system transmission bandwidth, 6 MHz, the signal functions as a disturbance wave and reduces the image quality of CATV.